Graphite Particles And Lithium Secondary Battery Using The Same As Negative Electrode

ABSTRACT

A method for forming a negative electrode for a lithium secondary battery, includes providing a paste comprising graphite particulates comprise assembled or bound graphite particles in each of which a plurality of flat-shaped particles are assembled or bound together so that the planes of orientation are not parallel to one another, and the mixture including 3 to 10 parts by weight of the organic binder per 100 parts by weight of the graphite particulates, a binder and a solvent, coating the paste on a current collector, drying the paste coated on the current collector to form a mixture of the graphite particulates and the binder, and integrating the mixture with the current collector by pressing to provide a density of the mixture of graphite particulates and organic binder of 1.5 to 1.9 g/cm 3 .

This application is a Divisional application of application Ser. No.09/230,889, filed Feb. 2, 1999, which is an Application under 35 USC 371of International application Serial No. PCT/JP97/02762, filed Aug. 7,1997.

TECHNICAL FIELD

This invention relates to a novel graphite particle, a process forproducing the same, a graphite paste using the graphite particle, anegative electrode for lithium secondary battery using the graphitepaste, a process for producing the negative electrode, and a lithiumsecondary battery. More particularly, this invention relates to alithium secondary battery suitable for use in portable instruments,electric cars, electric power storage, etc. and excellent in the rapidcharge-discharge characteristics, the cycle characteristics, etc., andto graphite particle for use as a negative electrode thereof, a processfor producing the graphite particle, a graphite paste using the graphiteparticle, a negative electrode for lithium secondary battery using thegraphite paste, and a process for producing the negative electrode.

BACKGROUND ART

As prior art graphite particles, natural graphite particle, artificialgraphite particle prepared by graphitization of coke, artificialgraphite particle prepared by graphitization of organic polymericmaterial, pitch and the like, graphite particles prepared by pulverizingthese graphites, etc. can be referred to. These graphite particles areput to use as a negative electrode for lithium secondary battery bymixing a graphite particle with an organic binder and an organic solventto prepare a graphite paste, coating a copper foil surface with thegraphite paste, and then evaporating the solvent. For instance, it isintended in JP-B 62-23433 to eliminate the problem of internalshort-circuit caused by lithium dendrite and to improve the cyclecharacteristics by using graphite as negative electrode.

However, in the natural graphite particle in which graphite crystals arewell grown and in the artificial graphites prepared by graphitization ofcoke, the interlaminar bonding force in the direction of c-axis ofcrystal is weaker than the bonding force in the crystal face direction,and therefore the bonding between graphite layers is broken uponpulverization, to form the so-called “flake graphite” having a largeaspect ratio. If the flake graphite particle having a great aspect ratiois kneaded together with a binder and coated onto a current collector toform an electrode, the flaky graphite particles are oriented in theplane direction of current collector. As its result, due to repeatedocclusion of lithium into graphite crystal and its release, a strainarises in the direction of c-axis, which causes an internal breakage ofelectrode. Thus, the cycle characteristics are deteriorated and, inaddition, the rapid charge-discharge characteristics tend to becomeworse.

Further, prior art graphite particles having a large crystallite size inthe face direction requires a long period of time for occlusion andrelease of lithium. Further, prior flaky graphite particles having ahigh aspect ratio have a great specific surface area. Thus, the lithiumsecondary battery obtained therefrom has a large irreversible capacityin the first cycle and, in addition, such graphite particles are poor inadhesiveness to current collector so that a large quantity of binder isneeded. If the adhesiveness to current collector is not good, thecurrent-collecting effect is not good and discharge capacity, rapidcharge-discharge characteristics and cycle characteristics aredeteriorated. Thus, it is desired to develop a graphite particleexcellent in the rapid charge-discharge characteristics and cyclecharacteristics, or small in the irreversible capacity in the firstcycle and excellent in cycle characteristics; or small in theirreversible capacity in the first cycle and capable of improving rapidcharge-discharge characteristics and cycle characteristics, in the formof a lithium secondary battery.

DISCLOSURE OF INVENTION

This invention provides graphite particles solving the problemsmentioned above and suitable for use as a negative electrode of alithium secondary battery excellent in rapid charge-dischargecharacteristics and cycle characteristics.

This invention further provides graphite particles suitable for use as anegative electrode of lithium secondary battery small in theirreversible capacity of the first cycle and excellent in cyclecharacteristics.

This invention further provides a process for producing graphiteparticles suitable for use as a negative electrode of a lithiumsecondary battery which is excellent in rapid charge-dischargecharacteristics and cycle characteristics, or small in the irreversiblecapacity of the first cycle and excellent in cycle characteristics, orsmall in the irreversible capacity of the first cycle and excellent inthe rapid charge-discharge characteristics and cycle characteristics.

This invention further provides a graphite paste suitable for use as anegative electrode of a lithium secondary battery which is excellent inrapid charge-discharge characteristics and cycle characteristics, orsmall in the irreversible capacity of the first cycle and excellent incycle characteristics, or small in the irreversible capacity of thefirst cycle and excellent in the rapid charge-discharge characteristicsand cycle characteristics.

This invention further provides a negative electrode of a lithiumsecondary battery which has a high capacity, and is excellent in therapid charge-discharge characteristics and cycle characteristics, orsmall in the irreversible capacity of the first cycle and excellent incycle characteristics, or small in the irreversible capacity andexcellent in the rapid charge-discharge characteristics and cyclecharacteristics, and a process for producing said negative electrode.

This invention further provides a lithium secondary battery which has ahigh capacity, and is excellent in the rapid charge-dischargecharacteristics and cycle characteristics, or small in the irreversiblecapacity of the first cycle and excellent in the cycle characteristics,or small in the irreversible capacity of the first cycle and excellentin the rapid charge-discharge characteristics and cycle characteristics.

The graphite particles of this invention have the followingcharacteristic features (1) to (6).

(1) Graphite particles obtained by assembling or binding together aplurality of flat-shaped particles so that the planes of orientation donot become parallel to one another.(2) Graphite particles in which aspect ratio of the graphite particle is5 or less.(3) Graphite particles in which specific surface area is 8 m²/g or less.(4) Graphite particles in which the size of crystallite in the directionof c-axis (the direction of thickness) of the crystal is 500 Å or moreand the size of crystallite in the direction of plane is 1,000 Å orless, both as measured by X ray broad angle diffraction.(5) Graphite particles in which pore volume of pores having a sizefalling in the range of 10² Å to 10⁶ Å is 0.4 to 2.0 cc/g based on theweight of graphite particle.(6) Graphite particles in which pore volume of pores having a sizefalling in the range of 1×10² Å to 2×10⁴ Å is 0.08 to 0.4 cc/g based onthe weight of graphite particle.

This invention further relates to a process for producing graphiteparticles described above characterized by mixing together an aggregate(raw material) which can be graphitized, graphite, a binder which can begraphitized, and 1 to 50% by weight of a graphitizing catalyst, followedby calcination and pulverization of the mixture.

This invention further relates to a graphite paste obtained by adding anorganic binder and a solvent to the above-mentioned graphite particlesor graphite particles produced by the above-mentioned process, andhomogenizing the mixture.

The negative electrode for the lithium secondary battery of thisinvention is produced by the use of the above-mentioned graphite paste,and has the following characteristic features (1) to (3).

(1) A negative electrode for a lithium secondary battery obtained bycoating the above-mentioned graphite paste onto a current collector andforming an integrated body.(2) A negative electrode for a lithium secondary battery obtained byintegrating a mixture of graphite particles and organic binder and acurrent collector, wherein the pressed and integrated mixture ofgraphite particles and organic binder has a density of 1.5 to 1.9 g/cm³.(3) A negative electrode for a lithium secondary battery obtained byintegrating a mixture of graphite particles and organic binder and acurrent collector, wherein the content of the organic binder is 3 to 20%by weight based on said mixture.

This invention further relates to a process for producing the negativeelectrode for lithium secondary battery of (2), characterized by adding1 to 50% by weight of a graphitizing catalyst to an aggregate which canbe graphitized or graphite and a binder which can be graphitized,homogenizing the mixture, calcining it, pulverizing it to obtaingraphite particles, adding and mixing an organic binder and a solvent tothe graphite particles, coating the mixture onto a current collector,and evaporating the solvent, followed by pressing and integration.

Further, this invention-relates to a lithium secondary batterycomprising a casing, a cover, at least one pair of negative and positiveelectrodes, said casing, cover and electrodes being disposed throughintermediation of separators, and an electrolytic solution provided inthe neighborhood of said casing, cover and electrodes, wherein saidnegative electrode is produced by the use of the above-mentionedgraphite particles.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are scanning electron microscopic photographs of thegraphite particles of this invention, wherein FIG. 1A is a photograph ofthe outer surface of the particle and FIG. 1B is a photograph of asection of the particle.

FIG. 2 is a partial sectional front view of a cylindrical lithiumsecondary battery.

FIG. 3 is a graph illustrating the relation between discharge capacityand charge-discharge cycle number.

FIG. 4 is a graph illustrating the relation between discharge capacityand charge-discharge current.

FIG. 5 is an outlined view of an example of this invention which is alithium secondary battery used for measurement of charge-dischargecharacteristics and irreversible capacity.

BEST MODE FOR CARRYING OUT THE INVENTION

The graphite particles of this invention can be classified into sixaccording to characteristic feature thereof.

The first graphite particle of this invention is a graphite particle inwhich a plurality of flat-shaped particles are assembled or boundtogether so that the planes of orientation thereof are not parallel toone another.

In this invention, the term “flat-shaped particle” means a particle soshaped as to have a major axis and a minor axis, namely so shaped as notto be a perfect sphere. For instance, this include scale-shaped ones,flake-shaped ones and a part of lump-shaped ones.

The term “planes of orientation are not parallel to one another” in aplurality of flat-shaped particles means a state that a plurality ofparticles are assembled so that the planes of orientation thereof arenot arranged in one direction, when a flat-shaped plane of each particleor, in other words, a plane closest to flatness, is taken as a plane oforientation.

From the viewpoint of constituent material, the individual, flat-shapedparticles are preferably made of a raw material (aggregate) which can begraphitized or of graphite.

In the graphite particles mentioned above, the flat-shaped particles areassembled or bound together. The term “bound together” means a statethat particles are made to adhere to each other through intermediationof a binder or the like, and the term “assembled” means a state thatparticles retain a form of gathered body due to shapes thereof or thelike, while each particles are not made to adhere to each other throughintermediation of binder or the like. From the viewpoint of mechanicalstrength, particles bound together are preferred.

The size of individual flat-shaped particles is preferably 1 to 100 μmand further preferably 1 to 50 μm, as expressed in terms of meanparticle diameter. It is preferable that this size of individualflat-shaped particles is ⅔ time or less as large as the mean particlesize of the assembled or bound together graphite particle. In onegraphite particle, the number of assembled or bound-together flat-shapedparticles is preferably 3 or more. In this invention, the mean particlesizes can be measured with a laser diffraction particle sizedistribution meter.

If such graphite particles are used as a negative electrode, thegraphite crystals do not readily undergo orientation onto a currentcollector, and lithium is readily occluded into the graphiteconstituting negative electrode and readily released therefrom. As aresult, the rapid charge-discharge characteristics and cyclecharacteristics of the lithium secondary electrode obtained therefromcan be improved.

FIGS. 1A and 1B are scanning electron microscopic photographs of anexample of the graphite particles of this invention, wherein FIG. 1A isa scanning electron microscopic photograph of the outer surface of thegraphite particle of this invention, and FIG. 18 is a scanning electronmicroscopic photograph of a section of the graphite particle. In FIG.1A, it can be observed that many flaky graphite particles are boundtogether so that the planes of orientation thereof are not parallel toone another, to form a graphite particle.

The second graphite particle of this invention is a graphite particlehaving an aspect ratio of 5 or less. When this graphite particle isused, orientation of particles on a current collector is difficult torealize, so that lithium is readily occluded and released similarly tothe above case.

The aspect ratio is preferably in the range of from 1.2 to 5. When theaspect ratio is smaller than 1.2, contact area between particlesdecreases, due to which conductivity decreases. For the same reason asabove, a more preferable range of aspect ratio is 1.3 or more.

On the other hand, the upper limit of the aspect ratio is 5, andpreferably 3 or less. When the aspect ratio is greater than 5, the rapidcharge-discharge characteristics tend to be deteriorated. Thus, the mostpreferable value of the aspect ratio is 1.3 to 3.

If the length of graphite particle in the direction of major axis isexpressed by A and that in the direction of minor axis is expressed byB, an aspect ratio is expressed by A/B. In this invention, aspect ratiois determined by magnifying graphite particles under a microscope,selecting 100 graphite particles at random, measuring A/B thereof, andcalculating mean value thereof.

Of the graphite particles having an aspect ratio of 5 or less, anassembly or bound material of graphite particles having a smaller sizeis preferable.

The third graphite particle of this invention is a graphite particlehaving a specific surface area of 8 m²/g or less. The specific area ispreferably 5 m²/g or less, more preferably 1.5-5 m²/g, and furtherpreferably 2-5 m²/g. By using such a graphite particle as a negativeelectrode, the rapid charge-discharge characteristics and cyclecharacteristics of the lithium secondary battery obtained therefrom canbe improved, and the irreversible capacity in the first cycle can bedecreased. If the specific surface area is greater than 8 m²/g, theirreversible capacity of the first cycle of the lithium secondarybattery obtained therefrom is high and the energy density is low, andfurther there is a problem that the preparation of negative electroderequires to use a large quantity of binder. On the other hand, if, thespecific surface area is smaller than 1.5 m²/g, the rapidcharge-discharge characteristics and cycle characteristics of thelithium secondary battery obtained therefrom tend to be deteriorated.The specific surface area can be measured by known methods such as BETmethod (nitrogen gas adsorption method) or the like.

The fourth graphite particle of this invention is a graphite particle inwhich, as measured by X ray broad angle diffraction, the size Lc ofcrystallite in the c-axis direction of crystal (002) is 500 Å or moreand the size La of crystallite in the plane direction (110) is 1,000 Åor less. By using such a graphite particle as a negative electrode, therapid charge-discharge characteristics and cycle characteristics of thelithium secondary battery obtained therefrom can be improved. The sizeLc of crystallite in the c-axis direction of crystal (002) is preferablyin the range of from 1,000 to 100,000 Å, provided that Lc (002)exceeding 3,000 Å cannot be determined accurately by means of X raybroad angle diffraction, and the size La of crystallite in the crystalplane direction (110) is preferably in the range of from 800 to 50 Å.

If size Lc (002) of crystallite in the c-axis direction is less than 500Å or if size La (110) of crystallite in the crystal plane direction isgreater than 1,000 Å, charge capacity becomes smaller.

In the fourth graphite particle, the interlaminar distance d (002) ofcrystal measured by X ray broad angle diffraction of graphite particleis preferably 3.38 Å or less and further preferably in the range of from3.37 to 3.35 Å. If the interlaminar distance d (002) of crystal exceeds3.38 Å, charge capacity tends to decrease.

The fifth graphite particle of this invention is characterized in thatthe pore volume of pores having a size falling in the range of 10² to10⁶ Å is 0.4 to 2.0 cc/g based on the weight of graphite particle. Byusing such a graphite particle as a negative electrode, the expansionand contraction of electrode upon charge and discharge are absorbed bythe pores of graphite particles; due to which the internal breakage ofelectrode can be suppressed, which results in an improvement of cyclecharacteristics of the lithium secondary battery obtained therefrom. Thepore volume of the pores having a size falling in the range of 10² to10⁶ Å is more preferably in the range of 0.4 to 1.5 cc/g, and furtherpreferably in the range of 0.6 to 1.2 cc/g. If the total pore volume isless than 0.4 cc/g, cycle characteristics are not good. If the totalpore volume is greater than 2.0 cc/g, a large quantity of binder isnecessary for integrating graphite particles and a current collector,which decreases the capacity of the lithium secondary battery produced.The pore volume can be determined by a pore diameter distributionmeasurement using the mercury-porosimeter method. Pore size can also bedetermined by a pore size distribution measurement using themercury-porosimeter method.

The sixth graphite particle of this invention is characterized in thatthe pore volume of the pores having a size falling in the range of 1×10²to 2×10⁴ Å is 0.08 to 0.4 cc/g based on the weight of graphite particle.If such a graphite particle is used as a negative electrode, theexpansion and contraction of electrode upon charge and discharge areabsorbed by the pores of graphite particle, due to which the internalbreakage of electrode can be suppressed, which results in an improvementof cycle characteristics of the lithium secondary battery obtainedtherefrom. The pore volume of the pores having a size falling in therange of 1×10² to 2×10⁴ Å is more preferably in the range of 0.1 to 0.3cc/g. If the pore volume of the pores having a size falling in thisrange is smaller than 0.08 cc/g, cycle characteristics are not good. Ifthe pore volume of the pores having a size falling in this range isgreater than 4 cc/g, a large quantity of binder is necessary forintegrating graphite particles and a current collector, which decreasesthe capacity of lithium secondary battery obtained therefrom. The porevolume of the pores having a size falling in this range can also bedetermined by a pore diameter distribution measurement by themercury-porosimeter method.

In the above-mentioned second to sixth graphite particles of thisinvention, it is preferable that the graphite particle has thecharacteristic feature of the first graphite particle, namely that thegraphite particle is a graphite particle in which a plurality offlat-shaped particles are assembled or bound together so that the planesof orientation are not parallel to one another. If such a graphiteparticle is used as a negative electrode, orientation of the graphitecrystals on a current collector cannot be realized readily and occlusionof lithium into negative electrode graphite and its release from thenegative electrode graphite are facilitated, and the rapidcharge-discharge characteristics and cycle characteristics of thelithium secondary battery obtained therefrom can further be improved.

It is also preferable that the first graphite particle and the third tosixth graphite particles of this invention have the characteristicfeature of the second graphite of this invention, namely that they havean aspect ratio of 5 or less, because thereby orientation of particleson a current collector is made difficult to realize and the occlusionand release of lithium are facilitated similarly to the above. Aspectratio of the graphite particle is more preferably 3 or less. Lower limitof the aspect ratio is preferably 1.2 or more and further preferably 1.3or more.

It is also preferable that the first and second graphite particles andthe fourth to sixth graphite particles of this invention have thecharacteristic feature of the third graphite particle of this invention,namely that they have a specific surface area of 8 m²/g or less, morepreferably 5 m²/g or less and further preferably 2 to 5 m²/g. There is atendency that, if the specific surface area increases, the irreversiblecapacity increases and energy density of the lithium secondary batteryprepared therefrom decreases. There is also a tendency that, if thespecific surface area increases, not only the irreversible capacity ofthe lithium secondary battery prepared therefrom increases, but thequantity of binder necessary for preparation of negative electrode alsoincreases.

Further, in the first to third graphite particles and the fifth andsixth graphite particles of this invention, the interlaminar distance d(002) of crystal measured by X ray broad angle diffraction of graphitepowder should preferably be 3.38 Å or less and more preferably 3.37 Å orless, because a smaller interlaminar distance d (002) gives a higherdischarge capacity. On the other hand, the size of crystallite in thec-axis direction Lc (002) should preferably be 500 Å or more and furtherpreferably 1,000 Å or more, because a greater Lc (002) gives a higherdischarge capacity.

Further, it is preferable that the first to fourth graphite particles ofthis invention have the characteristic feature of the fifth and sixthgraphite particles of this invention, namely that they have a porevolume corresponding to a pore of specified size, because thereby theexpansion and contraction of electrode upon charge and discharge can beabsorbed by the pores of graphite particles, due to which internalbreakage of electrode can be suppressed and as its result the cyclecharacteristic of the lithium secondary battery can be improved.

In this invention, the size of the first to sixth graphite particles ispreferably 1 to 100 μm and more preferably 0.10 to 50 μm, as expressedin terms of mean particle diameter.

Although the method for making the above-mentioned graphite particlesfulfil the above-mentioned characteristic features is not particularlylimited, such graphite particles can be obtained by adding and mixing 1to 50% by weight of a graphitizing catalyst into a mixture of a rawmaterial which can be graphitized or graphite and a binder which can begraphitized, followed by calcination and pulverization. By such aprocedure, pores are formed in the spaces from which the graphitizingcatalyst has been eliminated, and thereby good characteristic propertiescan be given to the graphite particle of this invention. The quantity ofthe graphitizing catalyst is preferably 3 to 20% by weight.

Further, each of the above-mentioned graphite particles can also beprepared by appropriately selecting the method for mixing graphite oraggregate with a binder, the mixing ratio such as quantity of binder,etc., and the conditions of pulverization after calcination.

As the raw material which can be graphitized, coke powder, carbonizedproduct of resins, etc. can be used, and any powdery materials may beused without limitation, so far as the powdery materials can begraphitized. Among these powdery materials, coke powders which caneasily be graphitized, such as needle coke and the like, are preferable.

As the graphite, natural graphite powder, artificial graphite powder andthe like can be used. Any graphite can be used so far as it is powdery.Preferably, the raw material which can be graphitized and the graphitehave a particle diameter smaller than that of the graphite particleproduced according to this invention.

As the graphitizing catalyst, metals such as iron, nickel, titanium,silicon, boron and the like, carbides thereof and oxides thereof can beused. Of these graphitizing catalysts, carbides and oxides of silicon orboron are preferred.

Mean particle diameter of the graphitizing catalyst is preferably 150 μmor less, more preferably 100 μm or less, and further preferably 50 μm orless. When the mean particle diameter exceeds 150 μm, growth of crystalstends to be uneven and discharge capacity tends to be uneven.

The graphitizing catalyst is added in an amount of 1 to 50% by weight,preferably 5 to 40% by weight and more preferably 5 to 30% by weight,based on the graphite particle obtained. If the amount of graphitizingcatalyst is less than 1% by weight, growth of graphite crystals isunsatisfactory and at the same time pore volume in the graphiteparticles tends to become small. On the other hand, if the amount ofgraphitizing catalyst is larger than 50% by weight, workability isdeteriorated and at the same time pore volume in the graphite particlestend to become too large.

As the binder, organic materials such as tar, pitch, thermosettingresins, thermoplastic resins and the like are preferable. The amount ofbinder to be compounded is preferably 5 to 80% by weight, morepreferably 10 to 80% by weight, and further preferably 15 to 80% byweight, based on flat-shaped raw material which can be graphitized orgraphite. If the amount of the binder is too large or too small, thereis a tendency that aspect ratio and specific surface area of thegraphite particle obtained become too great.

The method for mixing together an aggregate which can be graphitized ora graphite and a binder is not particularly limited, and the mixing iscarried out by means of a kneader. Preferably, the mixing is carried outat a temperature not lower than softening point of the binder.Concretely saying, the mixing is preferably carried out at 50 to 300° C.when the binder is pitch, tar or the like, and at 20 to 100° C. when thebinder is a thermo-setting resin.

Subsequently, the mixture obtained above is calcined to perform agraphitizing treatment. If desired, the mixture may be formed into adesired shape before the graphitizing treatment. Further, if desired,the formed mixture may be pulverized before graphitizing treatment toadjust the particle diameter to a desired value. The calcination ispreferably carried out under a condition where the mixture is difficultto oxidize. For instance, the calcination is carried out in anatmosphere of nitrogen or argon gas or in vacuum. Temperature of thegraphitization is preferably 2,000° C. or above, more preferably 2,500°C. or above, and further preferably 2,800° C. to 3,200° C.

If the temperature of graphitization is low, graphite crystals cannotgrow satisfactorily and the graphitizing catalyst tends to remain in thegraphite particles. If the graphitizing catalyst remains in the graphiteparticles prepared, discharge capacity decreases. If the temperature ofgraphitization is too high, sublimation of graphite can occur.

It is preferable to pulverize the thus obtained graphitized product inthe subsequent step. The method for pulverization of the graphitizedproduct is not particularly limited, but known means such as jet mill,vibration mill, pin mill, hammer mill and the like may be used. Meanparticle diameter after the pulverization is preferably 1 to 100 μm, andmore preferably 10 to 50 μm. If the mean particle diameter is too great,there is a tendency that irregularities can readily be formed on thesurface of electrode.

In this invention, the above-mentioned graphite particles 1 to 6 can beobtained via the steps mentioned above.

The graphite paste of this invention is produced by mixing the graphiteparticles mentioned above with an organic binder, a solvent, etc.

As said organic binder, polyethylene, polypropylene, ethylene-propyleneterpolymer, butadiene rubber, styrene-butadiene rubber, butyl rubber,polymeric compounds having a high ionic conductivity, and the like canbe used.

As said polymeric compounds having a high ionic conductivity,polyvinylidene fluoride, polyethylene oxide, polyepichlorohydrin,polyphosphazene, polyacrylonitrile and the like can be used.

of these organic binders, polymeric compounds having a high ionicconductivity are preferred, and polyvinylidene fluoride is especiallypreferred.

Mixing ratio between the graphite particles and the organic binder ispreferably 3 to 10 parts by weight of organic binder per 100 parts byweight of graphite particles.

The solvent is not particularly limited. Organic solvents such asN-methyl-2-pyrrolidone, dimethylformamide, isopropanol and the like areused.

The amount of the solvent is not particularly limited, but it may be anyamount so far as the graphite paste can be adjusted to a desiredviscosity. Preferably, 30 to 70% by weight of solvent is used based ongraphite paste.

The negative electrode for lithium secondary battery of this inventionis roughly classified into three types according to characteristicfeatures.

The first negative electrode for lithium secondary battery of thisinvention is characterized by using one of the above-mentioned graphiteparticles. This negative electrode for lithium secondary battery can beobtained by forming the graphite paste into a shape of sheet, pellet,etc.

The graphite paste is coated onto a current collector and integratedtogether with the current collector to form a negative electrode.

As the current collector, metallic current collectors such as a foil ora mesh of nickel, copper or the like can be used. The integration can beperformed by a molding method using a roll, a press or the like or bycombination of these means.

The second negative electrode for lithium secondary battery of thisinvention is characterized in that a mixture of graphite particles andan organic binder is integrated together with a current collector and,after the integration, the mixture of graphite particles and binder hasa density of 1.5 to 1.9 g/cm³, preferably 1.55 to 1.85 g/cm³, morepreferably 1.6 to 1.85 g/cm³, and further preferably 1.6 to 1.8 g/cm³.By enhancing the density of the mixture of graphite particles and binderconstituting the negative electrode of this invention, the lithiumsecondary battery obtained from the negative electrode can have anincreased energy density per volume. If density of the mixture ofgraphite particles and organic binder exceeds 1.9 g/cm³, the rapidcharge-discharge characteristics are deteriorated. If the density isless than 1.5 g/cm³, the lithium secondary battery obtained therefromhas a decreased energy density.

The graphite particle used in the second negative electrode for lithiumsecondary battery of this invention may be any graphite so far as itsdensity can be designed so as to fall in the above-mentioned range.Materials other than the above-mentioned graphite particles of thisinvention such as natural graphite and the like are also usable of allthese graphite particles, however, the above-mentioned graphiteparticles of this invention are especially preferred because the use ofthe graphite particles of this invention makes it possible to improvethe discharge capacity, rapid charge-discharge characteristics and cyclecharacteristics when density of negative electrode of lithium secondaryelectrode prepared therefrom is enhanced.

The kinds of organic binder, current collector and solvent used in thesecond negative electrode for lithium secondary battery of thisinvention and mixing ratios of these ingredients may be the same asthose in the first negative electrode for lithium secondary battery ofthis invention.

In order to make the density of the mixture of graphite particles andorganic binder after integration fall in the above-mentioned range, itis preferable to carry out the integration of current collector and themixture of graphite particles and organic binder while applying apressure. The pressure may be applied by means of a roll, a press, etc.

The third negative electrode for lithium secondary battery of thisinvention is characterized in that the amount of the organic binder is 3to 20% by weight and preferably 11 to 20% by weight based on the weightof the mixture of graphite particles and organic binder. By adjustingthe amount of the organic binder so as to fall in such a range,discharge capacity of the negative electrode for lithium secondarybattery per weight of the mixture can be made high. The amount of theorganic binder is more preferably 12 to 16% by weight based on theweight of the mixture of graphite particles and organic binder. If theamount of the organic binder is less than 3% by weight, bonding forcesbetween graphite particles and between graphite particle and currentcollector are weak, due to which resistance is high at interfacesthereof, conductivity of the lithium secondary battery prepared is low,and the discharge capacities per weight of graphite particles and perweight of the mixture of graphite particles and organic binder are low.Further, the graphite particles are expanded and contracted upon chargeand discharge. Accordingly, when charge and discharge are repeated, abreakage becomes readily occurring between graphite particles andbetween graphite particle and current collector, due to which cyclecharacteristics are deteriorated. On the other hand, when the amount ofthe organic binder exceeds 20% by weight, a large quantity of organicbinder of low conductivity exists between graphite particles and betweengraphite particle and current collector, due to which electricalconductivity of negative electrode decreases, discharge capacity perweight of graphite particle decreases, and as its result dischargecapacity per weight of the mixture of graphite particles and organicbinder decreases. Further, since the organic binder is not charged nordischarged, addition of an organic binder in an amount exceeding 20% byweight makes the amount of graphite particles in the mixture so small asless than 80% by weight, due to which the discharge capacity per weightof the mixture of graphite particles and organic binder becomes small.

The use of the above-mentioned graphite particles of this invention asthe graphite particle of the third negative electrode for lithiumsecondary battery of this invention is preferable because, when thenegative electrode of the lithium secondary battery prepared therefromis made to have a high density, discharge capacity, rapidcharge-discharge characteristics and cycle characteristics can beimproved thereby.

The kinds of organic binder, current collector and solvent used in thethird negative electrode for lithium secondary battery of thisinvention, the mixing ratio of these ingredients and the moldingconditions of the current collector and the mixture may be the same asthose in the first negative electrode for lithium secondary battery ofthis invention. Like in the second negative electrode for lithiumsecondary battery, the molding conditions are preferably selected so asto give a density of 1.5 to 1.9 g/cm³ to the integrated mixture ofgraphite particles and binder.

Each of the negative electrodes for lithium secondary battery isdisposed so as to confront a positive electrode through intermediationof a separator, after which an electrolytic solution is poured. By sucha procedure, there can be prepared a lithium secondary battery which ishigher in capacity, more excellent in rapid charge-dischargecharacteristics and cycle characteristics and smaller in irreversiblecapacity than prior lithium secondary batteries.

The material used as a positive electrode of the lithium secondarybattery of this invention is not particularly limited, and LiNiO₂,LiCoO₂, LiMn₂O₄ and the like can be used either alone or in the form ofmixture.

As the electrolytic solution, the so-called organic electrolyticsolutions prepared by dissolving a lithium salt such as LiClO₄, LiPF₆,LiAsF₆, LiBF₄, LiSO₃CF₃ and the like in a non-aqueous solvent such asethylene carbonate, diethyl carbonate, dimethoxyethane, dimethylcarbonate, tetrahydrofuran, propylene carbonate and the like can beused.

As the separator, for instance, unwoven cloths, cloths, micro-porousfilms and combinations thereof using a polyolefin such as polyethylene,polypropylene or the like as a main component can be used.

FIG. 2 illustrates a partial sectional front view of one example of acylindrical lithium secondary battery; wherein 1 is positive electrode,2 is negative electrode, 3 is separator, 4 is positive electrode tab, 5is negative electrode tab, 6 is positive electrode lid, 7 is batterycan, and 8 is gasket.

Next, this invention is explained by referring to examples and,according to the need, drawings.

Examples 1-7 are examples in which the first, second and third graphiteparticles of this invention are used as the graphite particle, and thefirst negative electrode for lithium secondary battery of this inventionis used as the negative electrode material for lithium secondarybattery.

Example 1 (1) Preparation of Graphite Particles

Seventy parts by weight of coke powder having a mean particle diameterof 10 μm, 20 parts by weight of tar pitch, 10 parts by weight of ironoxide and 20 parts by weight of coal tar were mixed together and stirredat 100° C. for one hour. The mixture was calcined at 2,800° C. in anatmosphere of nitrogen and then pulverized to obtain graphite particleshaving a mean particle diameter of 20 μm. According to a scanningelectron microscopic photograph (SEM photograph) of the graphiteparticles thus obtained, the graphite particles had a structure in whicha plurality of flat-shaped particles were bound together so that theplanes of orientation were not parallel to one another. One hundredparticles were selected at random from the graphite particles thusobtained, and a mean value of their aspect ratios was measured. As aresult, the mean value was 1.8. In a X ray broad angle diffraction ofthe graphite particles thus obtained, the interlaminar distance d (002)of the crystal was 3.360 Å, and the size of crystallite Lc (002) was1,000 Å or more. The specific surface area was 3.5 m²/g as measured byBET method.

(2) Preparation of Lithium Secondary Battery

A lithium secondary battery having the shape shown in FIG. 2 wasprepared in the following manner. As a positive electrode activematerial, 88% by weight of LiCoO₂ was used. As an electroconductivematerial, 7% by weight of a flaky natural graphite having ameans-particle diameter of 1 μm was used. As a binder, 5% by weight ofpolyvinylidene fluoride (PVDF) was used. To these materials was addedN-methyl-2-pyrrolidone (its amount was 50% by weight based on the paste,hereinafter the same), and the mixture was homogenized to obtain a pasteas a mixture for forming a positive electrode. In the same manner asabove, a negative electrode active material was prepared by adding 10%by weight of PVDF as a binder to 90 parts by weight of the graphitepowder obtained in (1). By adding thereto N-methyl-2-pyrrolidone (itsamount was 50% by weight of the paste, hereinafter the same) andhomogenizing the mixture, there was obtained a paste of a mixture forforming a negative electrode.

Subsequently, the paste of the mixture for forming a positive electrode,mentioned above, was coated onto both sides of an aluminum foil having athickness of 25 μm and dried in vacuum at 120° C. for one hour. Afterdryness, an electrode was press-formed therefrom by means of rollerpress so as to have a thickness of 190 μm. The amount of coating of themixture for forming a positive electrode per unit area was 49 mg/cm². Itwas cut into a size having a width of 40 mm and a length of 285 mm toprepare a positive electrode 1, provided that both terminal portions(each 10 mm in length) of the positive electrode 1 had no coating of themixture for forming a positive electrode, so that the aluminum foil wasexposed in these portions. To one of the exposed aluminum foil portions,positive tab 4 was contact-bonded by the ultrasonic bonding method.

On the other hand, the paste of mixture for forming a negative electrodewas coated on both sides of a copper foil having a thickness of 10 μmand vacuum-dried at 120° C. for one hour. After dryness, an electrodewas press-formed therefrom by means of a roller press and thickness wasadjusted to 175 μm. The coating amount of the mixture for forming anegative electrode per unit area was 20 mg/cm², which was cut into asize of 40 mm in width and 290 mm in length to prepare negativeelectrode 2. Like the positive electrode 1, both terminal portions, eachhaving a length of 10 mm, of the negative electrode 2 had no coating ofthe mixture for forming negative electrode, so that the copper foil wasexposed. To one of the exposed copper foil portions, a negativeelectrode tab 5 was contact-bonded by means of ultrasonic wave.

As separator 3, a polyethylene-made micro-porous film having a thicknessof 25 μm and a width of 44 mm was used. Then, as shown in FIG. 2,positive electrode 1, separator 3, negative electrode 2 and separator 3were successively piled, and the laminate thus obtained was rolled toprepare an electrode group. The electrode group was inserted into abattery can 7 of Single-3 size, a negative electrode tab 5 was bonded tothe can bottom by welding, and a squeezed part was provided for caulkinga positive electrode lid 6. Then, an electrolytic solution prepared bydissolving 1 mol/liter of lithium hexafluoro-phosphate in 1:1 mixture ofethylene carbonate and dimethyl carbonate (not shown in the drawing) waspoured into the battery can 7, a positive electrode tab 4 was bonded topositive electrode lid 6 by welding, and then the positive electrode lid6 was caulked to obtain a lithium secondary battery.

On the lithium secondary battery thus obtained, charge and dischargewere repeated at a charge-discharge current of 300 mA, a final chargevoltage of 4.15 V and a final discharge voltage of 2.8 V. Further, arapid charge-discharge was carried out while changing thecharge-discharge current in a range of from 300 mA to 900 mA. Theresults are shown in FIG. 3 and FIG. 4.

Example 2

Seventy parts by weight of coke powder having a mean particle diameterof 10 μm, 10 parts by weight of tarpitch, 2 parts by weight of ironoxide and 20 parts by weight of coal tar were mixed together and stirredat 100° C. for one hour. Subsequently, the mixture was calcined in anatmosphere of nitrogen at 2,800° C. and then pulverized to obtaingraphite particles having a mean particle diameter of 20 μm. Anexamination of the graphite particles thus obtained under an electronmicroscope revealed that a plurality of flat-shaped particles wereassembled or bound together so that the planes of orientation thereofwere not parallel to one another to form a graphite particle. Onehundred particles were at random selected from the graphite particles,and mean value of aspect ratios thereof was determined to obtain aresult of 4.8. X ray broad angle diffraction of the graphite particlethus obtained revealed that the interlaminar distance d (002) of thecrystal was 3.363 Å and the size of crystallite Lc (002) was 1,000 Å ormore. As measured by BET method, the specific area was 4.3 m²/g.

A lithium secondary battery was prepared from the graphite particlesthus obtained by the same procedure as in Example 1, and batterycharacteristics thereof were examined in the same manner as inExample 1. The results are shown in FIG. 3 and FIG. 4.

Example 3

A coke powder having a mean particle diameter of 20 μm was calcined inan atmosphere of nitrogen at 2,800° C. to obtain graphite particleshaving a mean particle diameter of 20 μm. The graphite particles thusobtained constituted a flaky graphite having a mean aspect ratio of 6, aspecific surface area of 11 m²/g, an interlaminar distance d (002) incrystal of 3.365 Å and a crystallite size Lc (002) of 800 Å.

The flaky graphite thus obtained was made into a lithium secondarybattery via the same processes as in Example 1, and the batterycharacteristics thereof were examined in the same manner as inExample 1. The results are shown in FIG. 3 and FIG. 4.

The lithium secondary batteries obtained in Examples 1, 2 and 3 of thisinvention were compared with one another on occlusion and release oflithium. The results were as mentioned below. FIG. 3 is a graphillustrating the relation between discharge capacity andcharge-discharge cycle number in a repeated charge-discharge test oflithium secondary battery. In FIG. 3, Curve 9 depicts the dischargecapacity of the lithium secondary battery obtained in Example 1; Curve10 expresses discharge capacity of the lithium secondary batteryobtained in Example 2; and Curve 11 expresses discharge capacity of thelithium secondary battery obtained in Example 3.

In FIG. 3, the lithium secondary battery obtained in Example 1 shows ahighest discharge capacity of 750 mAh, and the capacity decrease rate ofthe discharge capacity in the 500th cycle based on the highest capacityis 8%. The lithium secondary battery obtained in Example 2 shows ahighest discharge capacity of 720 mAh, and the capacity decrease rate ofthe discharge capacity in the 500th cycle based on the highest capacityis 12%. The lithium secondary battery obtained in Example 3 shows ahighest discharge capacity of 650 mAh, and the capacity decrease rate ofthe discharge capacity in the 510th cycle based on the highest capacityis 31%.

FIG. 4 illustrates the relation between charge-discharge current anddischarge capacity in a rapid charge-discharge test. Curve 12 depictsdischarge capacity of the lithium secondary battery obtained in Example1; Curve 13 expresses discharge capacity of the lithium secondarybattery obtained in Example 2; and Curve 14 expresses discharge capacityof the lithium secondary battery obtained in Example 3. At acharge-discharge current of 900 mA, the lithium secondary batteryobtained in Example 1 shows a discharge capacity of 630 mAh and thelithium secondary battery obtained in Example 2 shows a dischargecapacity of 520 mAh. On the other hand, the lithium secondary batteryobtained in Example 3 shows a discharge capacity of 350 mA. At acharge-discharge current of 300 mAh, capacity decrease rate based ondischarge capacity is 16% in the lithium secondary battery obtained inExample 1, 28% in that obtained in Example 2, and 46% in that obtainedin Example 3.

Based on the results of tests of Examples 1, 2 and 3, it has beenconfirmed that the lithium secondary batteries using the first, secondand third graphite particles of this invention are high in capacity andexcellent in the charge-discharge cycle characteristics, and have goodrapid charge-discharge characteristics.

Example 4

Fifty parts by weight of a coke powder having a mean particle diameterof 10 μm, 20 parts by weight of tar pitch, 10 parts by weight of siliconcarbide and 20 parts by weight of coal tar were mixed together andstirred at 100° C. for one hour. The mixture was calcined in anatmosphere of nitrogen at 2,800° C. and then pulverized to obtaingraphite particles having a mean particle diameter of 20 μm. One hundredparticles were selected therefrom at random, and mean aspect ratiothereof was measured to obtain a result of 1.5. As measured by BETmethod, 0.25 specific surface area of the graphite particles thusobtained was 2.9 m²/g. As measured by X ray broad angle diffraction,interlaminar distance d (002) of the crystal thereof was 3.360 Å, andcrystallite size Lc (002) was 1,000 Å or above. According to a scanningelectron microscopic photograph (SEM photograph), the graphite particlesthus obtained had a structure in which a plurality of flat-shapedparticles were assembled or bound together so that the planes oforientation were not parallel to one another.

Subsequently, 90% by weight of the graphite particle thus obtained waskneaded together with 10% by weight (weight of solid component) ofpolyvinylidene fluoride dissolved in N-methyl-2-pyrrolidone to obtain agraphite paste. The graphite paste was coated onto a rolled copper foilhaving a thickness of 10 μm, dried and compression-molded under asurface pressure of 490 MPa (0.5 ton/cm²) to obtain a sample electrode.Thickness of graphite particle layer and density thereof were adjustedto 75 μm and 1.5 g/cm³, respectively.

The sample electrode thus prepared was subjected to a constant currentcharge-discharge test by the 3-terminals method to evaluate itsperformance as a negative electrode for lithium secondary battery. FIG.5 is an outlined view of the lithium secondary battery. The sampleelectrode was evaluated by preparing an electrolytic solution 16consisting of LiPF₄ dissolved in 1:1 (by volume) mixture of ethylenecarbonate (EC) and dimethyl carbonate (DMC), concentration of LiPF₄ insaid solution being 1 mol/liter, introducing the resulting solution intoa glass cell 15 as shown in FIG. 5, laminating a sample electrode(negative electrode) 17, a separator 18 and a counter electrode(positive electrode) 19, and hanging a reference electrode 20 up-to-downto prepare a lithium secondary battery. Metallic lithium was used as thecounter electrode 19 and the reference electrode 20; and a micro-porouspolyethylene film was used as the separator 18.

Using the lithium secondary battery thus obtained, charging at aconstant current of 0.3 mA/cm² (per area of sample electrode) wascarried out until the voltage between sample electrode 17 and counterelectrode 19 reached 5 mV (Vvs. Li/Li⁺) and then discharging was carriedout until the voltage reached 1 V (Vvs. Li/Li⁺), and this cycle wasrepeated to make a test. Table 1 illustrates the charge capacity perunit weight of graphite particles, the discharge capacity per unitweight of graphite particles and the irreversible capacity in the firstcycle, and the discharge capacity per unit weight of graphite particlesin the 50th cycle. Further, as an evaluation of the rapidcharge-discharge characteristics, Table 2 illustrates the change indischarge capacity in an experiment of charging at a constant current of0.3 mA/cm² followed by discharging at a varied discharge current of 0.3,2.0, 4.0 and 6.0 mA/cm².

Example 5

Fifty parts by weight of coke powder having a mean particle diameter of10 μm, 10 parts by weight of tar pitch, 5 parts by weight of siliconcarbide and 10 parts by weight of coal tar were mixed together andstirred at 100° C. for one hour. The mixture was calcined in anatmosphere of nitrogen at 2,800° C. and pulverized to obtain graphiteparticles having a mean particle diameter of 20 μm. One hundredparticles were selected therefrom at random, and the mean value ofaspect ratio was calculated to obtain a result of 4.5. As measured byBET method, specific surface area of the graphite particles thusobtained was 4.9 m²/g. As measured by X ray broad angle diffraction, theinterlaminar distance d (002) of the graphite crystal was 3.362 Å, andthe size of crystallite Lc (002) was 1,000 Å or above. The graphiteparticles obtained herein had a structure in which a plurality offlat-shaped particles were assembled or bound together so that theplanes of orientation thereof were not parallel to one another.

Thereafter, the procedure of Example 4 was repeated to obtain a lithiumsecondary battery, which was then tested in the same manner as inExample 4. Table 1 illustrates the charge capacity per unit weight ofgraphite particles, the discharge capacity per unit weight of graphiteparticles and the irreversible capacity in the first cycle, and thedischarge capacity per unit weight of graphite particles in the 50thcycle. Further, as an evaluation of rapid charge-dischargecharacteristics, Table 2 illustrates the change in discharge capacity inan experiment of charging at a constant current of 0.3 mA/cm² followedby discharging at a varied discharge current of 0.3, 2.0, 4.0 and 6.0mA/cm².

Example 6

Fifty parts by weight of a coke powder having a mean particle diameterof 10 μm, 5 parts by weight of a tar pitch and 5 parts by weight of acoal tar were mixed together and stirred at 100° C. for one hour. Themixture was calcined in an atmosphere of nitrogen at 2,800° C. andpulverized to obtain graphite particles having a mean particle diameterof 20 μm. One hundred particles were selected therefrom at random, andmean value of aspect ratio thereof was determined to obtain a result of5. As measured by BET method, specific surface area of the graphiteparticle thus obtained was 6.3 m²/g. As measured by X ray broad anglediffraction, the interlaminar distance d (002) of the crystal was 3.368Å, and the size of crystallite Lc (002) was 700 Å. The graphiteparticles thus obtained had a structure in which a plurality offlat-shaped particles were assembled or bound together so that theplanes of orientation were not parallel to one another.

Thereafter, the procedure of Example 4 was repeated to prepare a lithiumsecondary battery, and it was tested in the same manner as in Example 4.Table 1 illustrates the charge capacity per unit weight of graphiteparticles, the discharge capacity per unit weight of graphite particlesand the irreversible capacity in the first cycle, and the dischargecapacity per unit weight of graphite particles in the 50th cycle.Further, as an evaluation of rapid charge-discharge characteristics,Table 2 illustrates the change in discharge capacity in an experiment ofcharging at a constant current of 0.3 mA/cm² followed by discharging ata varied discharge current of 0.3, 2.0, 4.0 and 6.0 mA/cm².

Example 7

A coke powder having a mean particle diameter of 22 μm was calcined inan atmosphere of nitrogen at 2,800° C. to obtain graphite particleshaving a mean particle diameter of 20 μm. The graphite particles thusobtained constituted a flaky graphite having a mean aspect ratio of 7and a specific surface area of 8.5 m²/g as measured by BET method. Theinterlaminar distance d (002) of the crystal was 3.368 Å and crystallitesize Lc (002) was 800 Å, as measured by X ray broad angle diffraction.

The procedure of Example 4 was repeated to obtain a lithium secondarybattery, and the battery performance was examined in the same manner asin Example 4. Table 1 illustrates the charge capacity per unit weight ofgraphite particles, the discharge capacity per unit weight of graphiteparticles and the irreversible capacity in the first cycle, and thedischarge capacity per unit weight of graphite particles in the 50thcycle. Further, as an evaluation of rapid charge-dischargecharacteristics, Table 2 illustrates the change in discharge capacity inan experiment of charging at a constant current of 0.3 mA/cm² followedby discharging at a varied discharge current of 0.3, 2.0, 4.0 and 6.0mA/cm².

TABLE 1 Example 4 Example 5 Example 6 Example 7 First cycle Charge 370375 363 366 capacity (mAh/g) Discharge 335 325 302 294 capacity (mAh/g)Irreversible 9.5 13.3 16.8 19.7 capacity (%) Discharge capacity of 320300 265 220 the 50th cycle (mAh/g)

TABLE 2 Example 4 Example 5 Example 6 Example 7 Charge At a capacitydischarge (mAh/g) current of: 0.3 mA/cm² 335 325 302 294 2.0 mA/cm² 330319 288 280 4.0 mA/cm² 312 292 245 219 6.0 mA/cm² 280 220 180 105

It is apparent from Tables 1 and 2 that the lithium secondary batteriesusing the first, second and third graphite particles of this inventionare higher in discharge capacity, smaller in the irreversible capacityof the first cycle and more excellent in cycle characteristics and rapidcharge-discharge characteristics than that of Example 7.

In Examples 8 to 11 presented below, there are studied the use of thefourth graphite particle of this invention as a graphite particle, andthe use of the first negative electrode material for lithium secondarybattery of this invention as a negative electrode material for lithiumsecondary battery.

Example 8 (1) Preparation of Graphite Particles

Fifty parts by weight of coke powder having a mean particle diameter of10 μm, 20 parts by weight of tar pitch, 12 parts by weight of iron oxidehaving a mean particle diameter of 65 μm and 18 parts by weight of coaltar were mixed together and stirred at 200° C. for one hour. The mixturewas calcined first at 800° C. and then at 2,800° C. in an atmosphere ofnitrogen, and pulverized to obtain graphite particles having a meanparticle diameter of 20 μm. According to a scanning electron microscopicphotograph (SEM photograph) of the graphite particles thus obtained, thegraphite particle had a structure in which a plurality of flat-shapedparticles were assembled or bound together so that the planes oforientation were not parallel to one another. On hundred particles wereselected at random, and mean value of aspect ratio was calculated toobtain a result of 1.7. As measured by X ray broad angle diffraction ofthe graphite particles, the interlaminar distance d (002) of the crystalwas 3.360 Å, and the crystallite size in the direction of plane La (110)was 720 Å and the crystallite size in the c-axis direction Lc (002) was1,800 Å.

(2) Preparation of Lithium Secondary Battery

The lithium secondary battery shown in FIG. 2 was prepared in thefollowing manner. A paste of a mixture for forming a positive electrodewas prepared by using 88% by weight of LiCoO₂ as a positive electrodeactive material, 7% by weight of flaky natural graphite having a meanparticle diameter of 1 μm as an electroconductive material and 5% byweight of polyvinylidene fluoride (PVDF) as a bindery adding theretoN-methyl-2-pyrrolidone and homogenizing the mixture. Similarly, a pasteof a mixture for forming a negative electrode was prepared by using 90%by weight of the graphite powder obtained in (1) as a negative electrodeactive material and 10% by weight of PVDF as a binder, addingN-methyl-2-pyrrolidone, and homogenizing the mixture.

Subsequently, the paste of the mixture for forming a positive electrodeobtained above was coated onto both sides of an aluminum foil having athickness of 25 μm and vacuum-dried at 120° C. for one hour. Afterdryness, press-molding using a roller press was carried out to adjustthe thickness to 190 μm. The coating amount of the mixture for forming apositive electrode per unit area was 49 mg/m². Cutting into a size of 40mm (width)×285 mm (length) gave positive electrode 1, provided that bothterminal portions, each having a length of 10 mm, of the positiveelectrode 1 were not coated with the mixture for forming a positiveelectrode, so that the aluminum foil was exposed there. A positiveelectrode tab 4 was contact-bonded to one of the terminal portions bythe method of ultrasonic bonding.

On the other hand, the paste of the mixture for forming a negativeelectrode obtained above was coated on both sides of a copper foilhaving a thickness of 10 μm and then vacuum-dried at 120° C. for onehour. After dryness, an electrode was formed by press-molding by meansof a roller press and thickness was adjusted to 175 μm. The coatingamount of the mixture for forming a negative electrode per unit area was20 mg/cm². Cutting into a size of 40 mm (width)×290 mm (length) gave anegative electrode 2. Like in the positive electrode 1, both terminalportions each having a length of 10 μm of the negative electrode 2 thusobtained were not coated with the mixture for forming a negativeelectrode, so that copper foil was exposed there. A negative electrodetab 5 was contact-bonded to one of these portions by the method ofultrasonic bonding.

As the separator 3, a micro-porous film made of polyethylene having athickness of 25 μm and a width of 44 mm was used. Subsequently, as shownin FIG. 1, positive electrode 1, separator 3, negative electrode 2 andseparator 3 were successively superposed and rolled up to form anelectrode group. The electrode group was inserted into a Single-3 sizebattery can 1, a negative tab 5 was bonded to the can bottom by welding,and a squeezed portion was provided for caulking a positive electrodelid 6. Then, an electrolytic solution (not shown in drawing) prepared bydissolving 1 mol/liter of lithium hexafluoro-phosphate in 1:1 mixture ofethylene carbonate and dimethyl carbonate was poured into the batterycan 7, a positive electrode tab 4 was bonded to the positive electrodelid 6, and then positive electrode lid 6 was caulked to obtain a lithiumsecondary battery.

Using the lithium secondary battery obtained above, a charge-dischargetest was repeatedly carried out at a charge-discharge current of 30.0mA, a final charge voltage of 4.15 V and a final discharge voltage of2.8 V. Further, a rapid charge-discharge test was carried out whilechanging the charge-discharge current in the range of 300 mA to 600 mA,and the discharge capacity per unit weight of graphite particles in thefirst cycle and the maintenance rate of discharge capacity per unitweight of graphite particles in the 100th cycle were measured. Theresults are shown in Table 3.

Example 9

Fifty five parts by weight of coke powder having a mean particlediameter of 10 μm, 22 parts by weight of tar pitch, 8 parts by weight ofboron nitride having a mean particle diameter of 25 μm and 15 parts byweight of coal tar were mixed together and stirred at 200° C. for onehour. The mixture was calcined first at 800° C. and then at 2,800° C. inan atmosphere of nitrogen and pulverized to obtain graphite particleshaving a mean particle diameter of 20 μm. According to a scanningelectron microscopic photograph (SEM photograph), the graphite particlesthus obtained had a structure in which flat-shaped particles wereassembled or bound together so that the planes of orientation were notparallel to one another. One hundred particles were selected at random,and mean value of aspect ratio was calculated to obtain a result of 1.5.According to X ray broad angle diffraction of the graphite particlesthus obtained, the interlaminar distance d (002) of the crystal was3.363 Å, and the crystallite size in the plane direction La (110) was560 Å and crystallite size in the c-axis direction Lc (002) was 1,760 Å.

A lithium secondary battery was prepared from the graphite particlesthus obtained by the same procedure as in Example 8, and batterycharacteristics thereof were examined in the same manner as in Example8. The results are shown in Table 3.

Example 10

Fifty seven parts by weight of coke powder having a mean particlediameter of 15 μm, 23 parts by weight of tar pitch and 20 parts byweight of coal tar were mixed together and stirred at 200° C. for onehour. The mixture was calcined first at 800° C. in an atmosphere ofnitrogen and then at 2,600° C. in an atmosphere of nitrogen, andpulverized to obtain graphite particles having a mean particle diameterof 20 μm. According to a scanning electron microscopic photograph, thegraphite particles thus obtained had a structure in which a plurality offlat-shaped particles were assembled or bound together so that theplanes of orientation were not parallel to one another. One hundredparticles were selected at random, and mean value of aspect ratiothereof was measured to obtain a result of 2.0. According to X ray broadangle diffraction of the graphite particles thus obtained, theinterlaminar distance d (002) of the crystal was 3.390 Å, and thecrystallite size in the plane direction La (110) was 460 Å andcrystallite size in the c-axis direction Lc (002) was 300 Å.

A lithium secondary battery was prepared from the graphite particlesthus obtained by the same procedure as in Example 8, and batterycharacteristics thereof were examined in the same manner as in Example9. The results are shown in Table 3.

Example 11

Graphite particles having a mean particle diameter of 20 μm wereprepared by repeating the procedure of Example 10, except that thecalcination was carried out at 3,000° C. According to a scanningelectron microscopic photograph, the graphite particles thus obtainedhad a structure in which a plurality of flat-shaped particles wereassembled or bound together so that the planes of orientation were notparallel to one another. One hundred particles were selected at random,and mean value of aspect ratio thereof was measured to obtain a resultof 2.2. According to X ray broad angle diffraction of the graphiteparticles thus obtained, the interlaminar distance d (002) of thecrystal was 3.357 Å, and the crystallite size in the plane direction La(110) was 1,730 Å and crystallite size in the c-axis direction Lc (002)was 2,050 Å.

A lithium secondary battery was prepared from the graphite particlesthus obtained by the same procedure as in Example 8, and batterycharacteristics thereof were examined in the same manner as inExample 1. The results are shown in Table 3.

TABLE 3 Exam- Exam- Exam- Exam- ple 8 ple 9 ple 10 ple 11 Charge-Discharge capacity 722 688 467 730 discharge (mAh) voltage 300Maintenance of 81 80 70 78 (mA) discharge capacity in the 100th cycle(%) Charge- Discharge capacity 688 669 359 380 discharge (mAh) voltage600 Maintenance of 79 78 64 66 (mA) discharge capacity in the 100thcycle

It is apparent from Table 3 that the lithium secondary batteries usingthe fourth graphite particle of this invention show a high dischargecapacity at a charge-discharge current of 300 mA and retain 70% or moreof the high discharge capacity even at an enhanced charge-dischargecurrent of 600 mA, exhibiting excellent rapid charge-dischargecharacteristics.

In Examples 12 to 15, there are studied the use of the fifth and sixthgraphite particles of this invention as a graphite particle and the useof the first negative electrode material for lithium secondary batteryof this invention as a negative electrode material for lithium secondarybattery.

Example 12

Forty parts by weight of a coke powder having a mean particle diameterof 5 μm, 25 parts by weight of tar pitch, 5 parts by weight of siliconcarbide having a mean particle diameter of 48 μm and 20 parts by weightof coal tar were mixed together and stirred at 200° C. for one hour. Themixture was calcined at 2,800° C. in an atmosphere of nitrogen andpulverized to obtain graphite particles having a mean particle diameterof 30 μm. Using Shimadzu Poresizer 9320, pore size distribution of thegraphite particles thus obtained was measured by the mercury porosimetermethod to find that the pore size covered a range of 10² to 10⁶ Å, andthe total pore volume per weight of graphite particle was 0.6 cc/g.Further, the pore volume of the pores having a pore size of 1×10² to2×10⁴ Å was 0.20 cc/g per weight of graphite particles. One hundredparticles were selected at random, and mean value of aspect ratiothereof was measured to obtain a result of 1.5. As measured by BETmethod, the specific surface area of graphite particles was 1.5 m²/g. Asmeasured by X ray broad angle diffraction of the graphite particles, theinterlaminar distance d (002) of the crystal was 3.362 Å, and thecrystallite size Lc (002) was 1,000 Å or above. Further, according to ascanning electron microscopic photograph (SEM photograph), the graphiteparticles had a structure in which a plurality of flat-shaped particleswere assembled or bound together so that the planes of orientation werenot parallel to one another.

Subsequently, a graphite paste was prepared by kneading 90% by weight ofthe graphite particles obtained above together with 10% by weight, asexpressed by weight of solid component, of a solution of polyvinylidenefluoride (PVDF) in N-methyl-2-pyrrolidone. The graphite paste was coatedonto a rolled copper foil having a thickness of 10 μm, dried, andcompression-molded under a surface pressure of 490 MPa (0.5 ton/cm²) toobtain a sample electrode. Thickness, of the graphite layer and densitywere adjusted to 90 μm and 1.6 g/cm³, respectively.

The sample electrode thus obtained was subjected to a constant currentcharge-discharge test by the 3-terminals method for the purpose ofevaluation as a negative electrode for lithium secondary battery. FIG. 5is an outlined view of the lithium secondary battery. Evaluation of thesample electrode was carried out as shown in FIG. 5 by introducing, intoglass cell 1, a 1 mol/liter solution of LiPF₆ in 1:1 (by volume) mixtureof ethylene carbonate (EC) and dimethyl carbonate (DMC) as anelectrolytic solution 2, disposing a laminate of sample electrode 3,separate 4 and counter electrode 5, and hanging a reference electrode 6up-to-down to form a lithium secondary-battery. As the counter electrode5 and reference electrode 6, metallic lithium was used. As the separator4, a micro-porous polyethylene film was used. Using the lithiumsecondary battery thus obtained, charging at a constant current of 0.5mA/cm² (per area of sample electrode) was carried out until the voltagebetween sample electrode 3 and counter electrode 5 reached 5 mV (Vvs.Li/Li⁺) and then discharging was carried out until the voltage reached 1V (Vvs. Li/Li⁺), and this cycle was repeated to make a test. Table 4illustrates the charge capacity and discharge capacity per unit weightof graphite particles in the first cycle, and the discharge capacity perunit weight of graphite particles in the 30th cycle.

Example 13

Fifty parts by weight of a coke powder having a mean particle diameterof 20 μm, 20 parts by weight of pitch, 7 parts by weight of siliconcarbide having a mean particle diameter of 48 μm and 10 parts by weightof coal tar were mixed together and stirred at 200° C. for one hour. Themixture was calcined at 2,800° C. in an atmosphere of nitrogen andpulverized to obtain graphite particles having a mean particle diameterof 30 μm. A pore size distribution measurement by the mercuryporosimeter method using Shimadzu Poresizer 9320 revealed that thegraphite particles thus obtained had pores covering a range of 10² to10⁶ Å, and the total pore volume per weight of graphite particles was1.5 cc/g. The pore volume of pores having a size falling in the range of1×10² to 2×10⁴ Å was 0.13 cc/g per weight of graphite particles. Onehundred graphite particles were selected therefrom at random and themean value of aspect ratio was measured to obtain a result of 2.3. Asmeasured by BET method, specific surface area of the graphite particleswas 3.6 m²/g. As measured by X ray broad angle diffraction of thegraphite particles, interlaminar distance d (002) of the crystal was3.361 Å, and the crystallite size Lc (002) was 1,000 Å or above. Thegraphite particles thus obtained had a structure in which a plurality offlat-shaped particles were assembled or bound together so that theplanes of orientation were not parallel to one another.

Thereafter, a lithium secondary battery was prepared by the sameprocedure as in Example 12, and tested in the same manner as in Example12. Table 4 illustrates the charge capacity and discharge capacity perunit weight of graphite particles in the first cycle and the dischargecapacity per unit weight of graphite particles in the 30th cycle.

Example 14

Meso Carbon Microbeads (manufactured by Kawasaki Steel Corporation,trade name KMFC) was calcined in an atmosphere of nitrogen at 2,800° C.to obtain graphite particles having a mean particle diameter of 25 μm.Using Shimadzu Poresizer 9320, pore size distribution of the graphiteparticles thus obtained was determined by mercury porosimeter method. Asa result, the graphite had pores covering a range of 10² to 10⁶ Å, andthe total pore volume per weight of graphite particles was 0.35 cc/g.The pore volume of the pores having a size falling in the range of 1×10²to 2×10⁴ Å was 0.06 cc/g per weight of graphite particles. One hundredgraphite particles were selected therefrom at random, and mean value ofaspect ratio was measured to obtain a result of 1. As measured by BETmethod, specific surface area of the graphite particles was 1-4 m²/g. Asmeasured by X ray broad angle diffraction of the graphite particles, theinterlaminar distance d (002) of the crystal was 3.378 Å and thecrystallite size Lc (002) was 500 Å.

A lithium secondary battery was prepared by repeating the procedure ofExample 12, and tested in the same manner as in Example 12. Table 4illustrates the charge capacity and discharge capacity per unit weightof the graphite particles in the first cycle, and the discharge capacityper unit weight of the graphite particles in the 30th cycle.

Example 15

Fifty parts by weight of coke powder having a mean particle diameter of5 μm, 10 parts by weight of pitch, 30 parts by weight of iron oxidehaving a mean particle diameter of 65 μm and 20 parts by weight of coaltar were mixed together and stirred at 200° C. for one hour. The mixturewas calcined at 2,800° C. in an atmosphere of nitrogen and thenpulverized to obtain graphite particles having a mean particle diameterof 15 μm. Using Shimadzu Poresizer 9320, pore size distribution of thegraphite particles thus obtained was determined by the mercuryporosimeter method to reveal that the graphite particles had porescovering a range of 10² to 10⁶ Å, the total pore volume per unit weightof graphite particles was 2.1 cc/g, and the pore volume of the poreshaving a size falling in the range of 1×10² to 2×10⁴ Å was 0.42 cc/g perweight of graphite particles. One hundred particles were selectedtherefrom at random, and mean value of aspect ratio was measured toobtain a result of 2.8. As measured by BET method, specific surface areaof the graphite particles was 8.3 m²/g. As measured by X ray broad anglediffraction of graphite particles, the interlaminar distance d (002) ofthe crystal was 3.365 Å, and the crystallite size Lc (002) was 1,000 Åor above.

Subsequently, a lithium secondary battery was prepared by the sameprocedure as in Example 12 and tested in the same manner as in Example12. Table 4 illustrates charge capacity and discharge capacity per unitweight of the graphite particles in the first cycle, and the chargecapacity per unit weight of graphite particles in the 30th cycle.

TABLE 4 Example Example Example Example 12 13 14 15 Charge capacity inthe 392 380 321 384 first cycle (mAh/g) Discharge capacity in the 353345 288 338 first cycle (mAh/g) Discharge capacity in the 346 338 159286 30th cycle (mAh/g)

It is apparent from Table 4 that the lithium secondary batteries usingthe fifth and sixth graphite particles of this invention have a highcapacity and are excellent in cycle characteristics.

In Examples 16 to 21, there are studied the use of the second negativeelectrode material for lithium secondary battery of this invention as anegative electrode material for lithium secondary battery.

Example 16

Fifty parts by weight of coke powder having a mean particle diameter of8 μm, 20 parts by weight of tar pitch, 5 parts by weight of siliconcarbide and 15 parts by weight of coal tar were mixed together andstirred at 100° C. for one hour. The mixture was calcined at 2,800° C.in an atmosphere of nitrogen and pulverized to obtain graphite particleshaving a mean particle diameter of 25 μm. One hundred particles wereselected therefrom at random and the mean value of aspect ratio wasmeasured to obtain a result of 1.5. As measured by BET method, specificsurface area of the graphite particles was 2.1 m²/g. As measured by Xray broad angle diffraction of the graphite particles, interlaminardistance d (002) of the crystal was 3.365 Å, and the crystallite size Lc(002) was 1,000 Å or above. The graphite particles thus obtained had astructure in which a plurality of flat-shaped particles were assembledor bound together so that the planes of orientation were not parallel toone another.

Subsequently, a graphite paste was prepared by kneading 90% by weight ofthe graphite particles obtained above together with 10% by weight, asexpressed by weight of solid component, of a solution of polyvinylidenefluoride (PVDF) in N-methyl-2-pyrrolidone. The graphite paste was coatedonto a rolled copper foil having a thickness of 10 μm, dried to removethe N-methyl-2-pyrrolidone, and compressed under a pressure of 30 MPa toobtain a sample electrode. Thickness of the graphite-PVDF mixture layerand density were adjusted to 80 μm and 1.55 g/cm³, respectively. Thesample electrode thus obtained was subjected to a constant currentcharge-discharge test by the 3-terminals method for the purpose ofevaluation as a negative electrode for lithium secondary battery. FIG. 5is an outlined view of the lithium secondary battery. Evaluation of thesample electrode was carried out as shown in FIG. 5 by introducing, intoglass cell 1, a 1 mol/liter solution of LiPF₆ in 1:1 (by volume) mixtureof ethylene carbonate (EC) and dimethyl carbonate (DMC) as anelectrolytic solution 2, disposing a laminate of sample electrode 3,separate 4 and counter electrode 5, and hanging a reference electrode 6up-to-down to form a lithium secondary battery. As the counter electrode5 and reference electrode 6, metallic lithium was used. As the separator4, a micro-porous polyethylene film was used.

Using the lithium secondary battery thus obtained, charging was carriedout at a constant current of 0.2 mA/cm² (per area of the graphiteparticle-PVDF mixture in sample electrode) until the voltage betweensample electrode 3 and counter electrode 5 reached 5 mV (Vvs. Li/Li⁺)and discharging was carried out until the voltage reached 1 V (Vvs.Li/Li⁺). Although this cycle was repeated 50 times to make a test, nodecrease in discharge capacity was noticeable. Further, as an evaluationof rapid charge-discharge characteristics, charging was carried out at aconstant current of 0.3 mA/cm² and then discharging was carried out at avaried discharging current of 0.5, 2.0, 4.0 and 6.0 mA/cm². The relationbetween discharge capacity and volume of graphite particle-PVDF mixtureis shown in Table 5.

Example 17

A sample electrode was prepared by repeating the procedure of Example16, except that the compression force under press was altered to 40 MPa.In the sample electrode thus obtained, thickness of the graphiteparticle-PVDF mixture was 80 μm, and density thereof was 1.63 g/cm³.

Subsequently, a lithium secondary battery was prepared by repeating theprocedure of Example 16, and a test was carried out in the same manneras in Example 16.

As a result, no decrease in discharge capacity was noticeable. Further,as an evaluation of rapid charge-discharge characteristics, charging wascarried out at a constant current of 0.3 mA/cm² and then discharging wascarried out at a varied discharge current of 0.5, 2.0, 4.0 and 6.0mA/cm². The discharge capacities were as shown in Table 5.

Example 18

A sample electrode was prepared by the same processes as in Example 1,except that the compression force of press was altered to 80 MPa. In thesample electrode thus obtained, thickness of the mixture of graphiteparticles and PVDF was 80 μm and density thereof was 1.75 g/cm³.

Subsequently, a lithium secondary battery was prepared by the sameprocesses as in Example 16, and tested in the same manner as in Example16. As a result, no decrease in discharge capacity was observed. Forevaluating the rapid charge-discharge characteristics, charging wascarried out at a constant current of 0.3 mA/cm² and discharging wascarried out at a varied discharge current of 0.5, 2.0, 4.0 and 6.0mA/cm². The discharge capacities were as shown in Table 5.

Example 19

A sample electrode was prepared by the same procedure as in Example 16,except that the compression force of press was altered to 100 MPa. Inthe sample electrode thus obtained, thickness of the mixture of graphiteparticles and PVDF was 80 μm and density thereof was 1.85 g/cm³.

Subsequently, a lithium secondary battery was prepared by the sameprocedure as in Example 16, and tested in the same manner as in Example16. As a result, no decrease in discharge capacity was observed. Forevaluating the rapid charge-discharge characteristics, charging wascarried out at a constant current of 0.3 mA/cm² and discharging wascarried out at a varied discharge current of 0.5, 2.0, 4.0 and 6.0mA/cm². The discharge capacities were as shown in Table 5.

Example 20

A sample electrode was prepared by the same procedure as in Example 16,except that the compression force of press was altered to 20 MPa. In thesample electrode thus Obtained, thickness of the mixture of graphiteparticles and PVDF was 80 μm and density thereof was 1.45 g/cm².

Subsequently, a lithium secondary battery was prepared by the sameprocedure as in Example 16, and tested in the same manner as in Example16. As a result, no decrease in discharge capacity was observed. Forevaluating the rapid charge-discharge characteristics, charging wascarried out at a constant current of 0.3 mA/cm² and discharging wascarried out at a varied discharge current of 0.5, 2.0, 4.0 and 6.0mA/cm². The discharge capacities were as shown in Table 5.

Example 21

A sample electrode was prepared by the same procedure as in Example 16,except that the compression force of press was altered to 140 MPa. Inthe sample electrode thus obtained, thickness of the mixture of graphiteparticles and PVDF was 80 μm and density thereof was 1.93 g/cm³.

Subsequently, a lithium secondary battery was prepared by the sameprocedure as in Example 16, and tested in the same manner as in Example16. As a result, a 15.7% decrease of discharge capacity was observed.For evaluating the rapid charge-discharge characteristics, charging wascarried out at a constant current of 0.3 mA/cm² and discharging wascarried out at a varied discharge current of 0.5, 2.0, 4.0 and 6.0mA/cm². The discharge capacities were as shown in Table 5.

TABLE 5 Example Example Example Example Example Example 16 17 18 19 2021 Density of graphite 1.55 1.63 1.75 1.85 1.45 1.93 particle/organicbinder mixture (g/cm³) Discharge At a discharge 542 562 570 587 509 527capacity current of: (mAh/cm²) 0.5 mA/cm² 2.0 mA/cm² 524 546 553 566 493481 4.0 mA/cm² 500 522 528 512 473 416 6.0 mA/cm² 468 484 478 470 442360

As shown in Table 5, it is apparent that the lithium secondary batteriesusing the second negative electrode for lithium secondary battery ofthis invention are high in discharge capacity and excellent in rapidcharge-discharge characteristics.

In Examples 22 to 29, there is studied a use of the third negativeelectrode material for lithium secondary battery of this invention as anegative electrode material for lithium secondary battery.

Example 22

Fifty parts by weight of coke powder having a mean particle diameter of10 μm, 20 parts by weight of tar pitch, 5 parts by weight of siliconcarbide and 15 parts by weight of coal tar were mixed together andstirred at 100° C. for one hour. The mixture was calcined in anatmosphere of nitrogen at 3,000° C. and pulverized to obtain graphiteparticles having a mean particle diameter of 25 μm. One hundredparticles were selected therefrom at random, and mean value of aspectratio was measured to obtain a result of 1.3. As measured by BET method,specific surface area of the graphite particles thus obtained was 1.9m²/g. As measured by X ray broad angle diffraction of the graphiteparticles, the interlaminar distance d (002) of the crystal was 3.36 Å,and the crystallite size Lc (002) was 1,000 Å or above. Further,according to a scanning electron microscopic photograph (SEM photograph)of the graphite particles thus obtained, the graphite particles had astructure in which a plurality of flat-shaped particles were assembledor bound together so that the planes of orientation were not parallel toone another.

Subsequently, 89% by weight of the graphite particles thus obtained waskneaded together with 11% by weight, as expressed by the weight of solidcomponent, of a solution of polyvinylidene fluoride (PVDF) inN-methyl-2-pyrrolidone to obtain a graphite paste. The graphite pastewas coated onto a rolled copper foil having a thickness of 10 μm, dried,and compressed by means of rollers to obtain a sample electrode in whichthickness of the graphite particle-PVDF mixture was 80 μm and densitythereof was 1.5 g/cm³.

The sample electrode thus prepared was subjected to a constant currentcharge-discharge test by 3-terminals method to evaluate its performanceas a negative electrode for lithium secondary battery. FIG. 5 is anoutlined view of the lithium secondary battery. The sample electrode wasevaluated by preparing an electrolytic solution 2 consisting of LiPFdissolved in 1:1 (by volume) mixture of ethylene carbonate (EC) anddimethyl carbonate (DMC) so that concentration of said solution came to1 mol/liter, introducing the resulting solution into a glass cell 1 asshown in FIG. 5, laminating a sample electrode 3, a separator 4 and acounter electrode 5, and hanging a reference electrode 6 up-to-down toprepare a lithium secondary battery. Metallic lithium was used as thecounter electrode 5 and the reference electrode 6; and a micro-porouspolyethylene film was used as the separator 4. Using the lithiumsecondary battery thus obtained, charging at a constant current of 0.3mA/cm² (per area of graphite particle-PVDF mixture in the sampleelectrode) was carried out until the voltage between sample electrode 3and counter electrode 5 reached 5 MV (Vvs. Li/Li⁺) and then dischargingwas carried out at a constant current of 0.3 mA/cm² until the voltagereached 1 V (Vvs. Li/Li⁺), and this cycle was repeated to make a test.Table 6 illustrates the discharge capacity per unit weight of graphiteparticles, the discharge capacity per unit weight of graphiteparticle-PVDF mixture, and the discharge capacity per unit weight ofgraphite particle-PVDF mixture in the 50th cycle. Further, as anevaluation of the rapid charge-discharge characteristics, Table 6 alsoillustrates the discharge capacity in an experiment of charging at aconstant current of 0.3 mA/cm² followed by discharging at a current of3.0 mA/cm².

Example 23

A graphite paste was prepared by kneading 87% by weight of the graphiteparticles obtained in Example 22 together with 13% by weight, asexpressed by the weight of solid component, of polyvinylidene fluoride(PVDF) dissolved in N-methyl-2-pyrrolidone. Thereafter, the procedure ofExample 22 was repeated to obtain a sample electrode in which thicknessof the graphite particle-PVDF mixture layer was 80 μm and densitythereof was 1.5 g/cm³.

Then, a lithium secondary battery was prepared by the same procedure asin Example 22, and tested in the same manner as in Example 22. Theresults are shown in Table 6.

Example 24

A graphite paste was prepared by kneading 85% by weight of the graphiteparticles obtained in Example 22 together with 15% by weight, asexpressed by the weight of solid component, of polyvinylidene fluoride(PVDF) dissolved in N-methyl-2-pyrrolidone. Thereafter, the procedure ofExample 22 was repeated to obtain a sample electrode in which thicknessof the graphite particle-PVDF mixture layer was 80 μm and densitythereof was 1.5 g/cm³.

Then, a lithium secondary battery was prepared by the same procedure asin Example 22, and tested in the same manner as in Example 22. Theresults are shown in Table 6.

Example 25

A graphite paste was prepared by kneading 82% by weight of the graphiteparticles obtained in Example 22 together with 18% by weight, asexpressed by the weight of solid component, of polyvinylidene fluoride(PVDF) dissolved in N-methyl-2-pyrrolidone. Thereafter, the procedure ofExample 22 was repeated to obtain a sample electrode in which thicknessof the graphite particle-PVDF mixture layer was 80 μm and densitythereof was 1.5 g/cm³.

Then, a lithium secondary battery was prepared by the same procedure asin Example 22, and tested in the same manner as in Example 22. Theresults are shown in Table 6.

Example 26

A graphite paste was prepared by kneading 80% by weight of the graphiteparticles obtained in Example 22 together with 20% by weight, asexpressed by the weight of solid component, of polyvinylidene fluoride(PVDF) dissolved in N-methyl-2-pyrrolidone. Thereafter, the procedure ofExample 22 was repeated to obtain a sample electrode in which thicknessof the graphite particle-PVDF mixture layer was 80 μm and density was1.5 g/cm³.

Then, a lithium secondary battery was prepared by the same procedure asin Example 22, and tested in the same manner as in Example 22. Theresults are shown in Table 6.

Example 27

A graphite paste was prepared by kneading 92% by weight of the graphiteparticles obtained in Example 22 together with 8% by weight, asexpressed by the weight of solid component, of polyvinylidene fluoride(PVDF) dissolved in N-methyl-2-pyrrolidone. Thereafter, the procedure ofExample 22 was repeated to obtain a sample electrode in which thicknessof the graphite particle-PVDF mixture layer was 80 μm and densitythereof was 1.5 g/cm³.

Then, a lithium secondary battery was prepared by the same procedure asin Example 22, and tested in the same manner as in Example 22. Theresults are shown in Table 6.

Example 28

A graphite paste was prepared by kneading 97.5% by weight of thegraphite particles obtained in Example 22 together with 2.5% by weight,as expressed by the weight of solid component, of polyvinylidenefluoride (PVDF) dissolved in N-methyl-2-pyrrolidone. Thereafter, theprocedure of Example 22 was repeated to obtain a sample electrode inwhich thickness of the graphite particle-PVDF mixture layer was 80 μmand density was 1.5 g/cm³.

Then, a lithium secondary battery was prepared by the same procedure asin Example 22, and tested in the same manner as in Example 22. Theresults are shown in Table 6.

Example 29

A graphite paste was prepared by kneading 78% by weight of the graphiteparticles obtained in Example 22 together with 22% by weight, asexpressed by the weight of solid component, of polyvinylidene fluoride(PVDF) dissolved in N-methyl-2-pyrrolidone. Thereafter, the procedure ofExample 22 was repeated to obtain a sample electrode in which thicknessof the graphite particle-PVDF mixture layer was 80 μm and densitythereof was 1.5 g/cm³.

Then, a lithium secondary battery was prepared by the same procedure asin Example 22, and tested in the same manner as in Example 22. Theresults are shown in Table 6.

TABLE 6 Example Example Example Example Example Example Example Example22 23 24 25 26 27 28 29 Content of PVDF (%) based on 10 13 15 18 20 82.5 22 graphite particle/PVDF (organic binder) mixture Dischargecapacity (mAh/g) 325 338 355 359 363 320 275 335 per unit weight ofgraphite particles (at discharge current of 0.3 mA/cm²) Dischargecapacity (mAh/g) 293 294 302 294 290 294 267 261 per unit weight ofgraphite particle/PVDF mixture (at discharge current of 0.3 mA/cm²)Discharge capacity (mAh/g) 287 290 295 292 286 265 134 252 per unitweight of graphite particle/PVDF mixture in the 50th cycle (at dischargecurrent of 0.3 mA/cm²) Discharge capacity (mAh/g) 267 282 280 278 271250 160 232 per unit weight of graphite particle/PVDF mixture (atdischarge current of 3.0 mA/cm²)

As shown in Table 6, it is apparent that the third lithium secondarybattery of this invention is high in capacity and excellent in rapidcharge-discharge characteristics and cycle characteristics.

INDUSTRIAL APPLICABILITY

The graphite particles of this invention are suitable for use in lithiumsecondary batteries excellent in rapid charge-discharge characteristicsand cycle characteristics.

Further, the graphite particles of this invention are suitable for usein lithium secondary batteries small in the irreversible capacity in thefirst cycle and excellent in cycle characteristics.

Further, according to the process of this invention for producinggraphite particles, there can be obtained graphite particles suitablefor use in lithium secondary batteries excellent in rapidcharge-discharge characteristics and cycle characteristics or lithiumsecondary batteries small in the irreversible capacity of the firstcycle and excellent in cycle characteristics or lithium secondarybatteries small in the irreversible capacity of the first cycle andexcellent in rapid charge-discharge characteristics and cyclecharacteristics.

Further, the graphite paste of this invention is suitable for use inlithium secondary batteries excellent in rapid charge-dischargecharacteristics and cycle characteristics, or lithium secondarybatteries small in the irreversible capacity of the first cycle andexcellent in cycle characteristics, or lithium secondary batteries smallin the irreversible capacity of the first cycle and excellent in rapidcharge-discharge characteristics and cycle characteristics.

Further, the negative electrode material for lithium secondary batteryof this invention and the process for production thereof are suitablyapplicable to lithium secondary batteries high in capacity and excellentin rapid charge-discharge characteristics and cycle characteristics, orlithium secondary batteries small in the irreversible capacity of thefirst cycle and excellent in cycle characteristics, or lithium secondarybatteries small in the irreversible capacity of the first cycle andexcellent in rapid charge-discharge characteristics and cyclecharacteristics.

Further, the lithium secondary batteries of this invention are high incapacity and excellent in rapid charge-discharge characteristics andcycle characteristics, or small in the irreversible capacity of thefirst cycle and excellent in cycle characteristics, or small in theirreversible capacity of the first cycle and excellent in rapidcharge-discharge characteristics and cycle characteristics.

1. A method for forming a negative electrode for a lithium secondarybattery, comprising: providing a paste comprising graphite particulatescomprise assembled or bound graphite particles in each of which aplurality of flat-shaped particles are assembled or bound together sothat the planes of orientation are not parallel to one another, andwherein said mixture comprises 3 to 10 parts by weight of said organicbinder per 100 parts by weight of said graphite particulates, a binderand a solvent; coating the paste on a current collector; drying thepaste coated on the current collector to form a mixture of the graphiteparticulates and the binder; and integrating the mixture with thecurrent collector by pressing to provide a density of the mixture ofgraphite particulates and organic binder of 1.5 to 1.9 g/cm³.
 2. Themethod according to claim 1 wherein the content of the organic binder is11 to 20% by weight based on the mixture of the graphite particles andthe organic binder.
 3. The method according to claim 1 wherein thecontent of the organic binder is 3 to 20% by weight based on the mixtureof the graphite particles and the organic binder.
 4. The methodaccording to claim 1 wherein an aspect ratio of the graphite particlesis 5 or less.
 5. The negative electrode for a lithium secondary batteryformed by the method according to claim
 1. 6. The negative electrode fora lithium secondary battery formed by the method according to claim 2.7. The negative electrode for a lithium secondary battery formed by themethod according to claim
 3. 8. The negative electrode for a lithiumsecondary battery formed by the method according to claim
 4. 9. Alithium secondary battery comprising a casing, a cover and at least onepair of negative and positive electrodes, said casing, cover andelectrodes being disposed through intermediation of separators, and anelectrolytic solution disposed in the surroundings thereof, wherein saidnegative electrode is the negative electrode of claim
 5. 10. A lithiumsecondary battery comprising a casing, a cover and at least one pair ofnegative and positive electrodes, said casing, cover and electrodesbeing disposed through intermediation of separators, and an electrolyticsolution disposed in the surroundings thereof, wherein said negativeelectrode is the negative electrode of claim
 6. 11. A lithium secondarybattery comprising a casing, a cover and at least one pair of negativeand positive electrodes, said casing, cover and electrodes beingdisposed through intermediation of separators, and an electrolyticsolution disposed in the surroundings thereof, wherein said negativeelectrode is the negative electrode of claim
 7. 12. A lithium secondarybattery comprising a casing, a cover and at least one pair of negativeand positive electrodes, said casing, cover and electrodes beingdisposed through intermediation of separators, and an electrolyticsolution disposed in the surroundings thereof, wherein said negativeelectrode is the negative electrode of claim 8.